Why does formation of bonds release energy

Some of the energy released by each bond that is formed in making carbon dioxide and water is used to break more bonds in the methane and oxygen molecules. In this way, the reaction becomes self-sustaining as long as methane and oxygen continue to be supplied.

The igniter can be turned off. If breaking bonds did not require energy, then fuels would not need an ignition device to start burning. They would just start burning on their own. The presence of spark plugs in your car attests to the fact that breaking chemical bonds requires energy. Note that the combustion of methane actually involves many smaller steps, so the equation above could be expanded out into even more detail.

Biologists often talk about energy being made available by the breakdown of sugar, implying that the breaking of chemical bonds in the sugar molecules releases energy. And yet in chemistry we learn that energy is released, not when chemical bonds are broken , but when they are formed. In fact, respiration supplies energy, not by the breaking of bonds in the substrate, but by the formation of strong bonds in the products.

However, the overall result of the process is to yield energy, and it is in this sense that biologists talk about the breakdown of sugar giving energy. The total energy input or output of a reaction equals the energy released in forming new bonds minus the energy used in breaking the original bonds. If it takes more energy to break the original bonds than is released when the new bonds are formed, then the net energy of the reaction is negative.

This means that energy must be pumped into the system to keep the reaction going. We can imagine the hydrogen atoms are like a ball rolling on a hill shaped like the potential curve. You can see that it would increase in speed as it goes down the hill, then slow down and move back as it went up the "hill".

But here is the important point: If the ball was at the bottom of the curve, you would have to add energy to move it up the hill.

You would have to add energy to break this chemical bond. Let's go back to the example of hydrogen and oxygen. If you get this reaction started, you do indeed get a lot of energy. But this energy does not come from the hydrogen-hydrogen bond, nor does it comes from the oxygen-oxygen bond. The energy comes from the formation of the hydrogen-oxygen bonds in the water.

Perhaps another energy sketch would help. Suppose I represent the energy of the gases and the water with the following:. Moving that ball to the lower part of the curve the water part requires a little bit of energy, but you get a lot back.

She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Properties of Ionic and Covalent Compounds. Enthalpy Change Definition in Science. Fusion Definition Physics and Chemistry. Covalent or Molecular Compound Properties. Understanding Endothermic and Exothermic Reactions. Are Glow Sticks Endothermic or Exothermic?

In an endothermic reaction, the products are at a higher energy than the reactants. Energy changes Breaking and making bonds During a chemical reaction: bonds in the reactants are broken new bonds are made in the products Energy is absorbed to break bonds.

If more heat energy is released when making the bonds than was taken in, the reaction is exothermic If more heat energy was taken in when making the bonds than was released, the reaction is endothermic.